1. Technical Field
The present invention relates to a method and apparatus for recording and reading information on a medium by using changes in optical characteristics. More particularly, the invention relates to optical disk apparatuses.
2. Background Art
A method of increasing the areal density of an optical disc is known from Japan Journal of Applied Physics, Volume 35, 1996, pp.437-442. In this method, called SCIPER (Single Carrier Independent Pit Edge Recording), marks are recorded on the disk surface at equal intervals. These positions of the front and rear edges of each mark are independently varied. The positional changes are measured at specific detection points in the form of changes in multi-levels at a particular detection point to read out the recorded information.
Referring to FIG. 1, the principle of the SCIPER method according to the prior art will be described. Groups of prepits 102 to 107 are formed along a track center 101 on the disc surface. Each group includes a pair of prepits 103 and 104 or a pair of prepits 105 and 106, each prepit being disposed on either side of the track center 101, for detecting a tracking signal. It also includes prepits 102 and 107 disposed at the track center for providing a clock. A data recording area, or data block, is disposed between the groups of prepits. The data block is further divided into a plurality of data cells 108 with a length P. Each data cell contains a mark 110, 111, 112. The mark is modulated by moving its front and rear edges forward or backward from a specific center position by distances which are integer multiples of a specific interval Δ, such that the edge positions represent information.
A minimum mark length Lmin of a mark formed is chosen such that when reading the recorded information by means of a reading spot 109, a readout signal picked up from the front edge is not influenced by the rear edge, i.e., such that there is no interference between the front and rear edges. A maximum mark length Lmax is selected such that the gap between the maximum-length mark in a data cell and the maximum-length mark in an adjacent data cell is equal to the minimum mark length Lmin, so that the signals from the front and rear, or the rear and front, edges of the two marks do not interfere with each other. User data is associated with a number (n+1)×(n+1) of combinations of edge positions, where n is the number of divisions of the position that each of the front and rear edges can take in units of interval Δ. In order to increase the recording density, the number of divisions n in units of interval Δ has to be increased.
FIG. 2 illustrates how the information stored at the front and rear edges of a mark is read. When the edge positions are not shifted, that is during non-modulation, the mark has its front edge positioned at 201 and rear edge at 202, with a mark length of Lo. The front edge position is varied independently based on its center at position 201 by an integer multiple of an interval Δ, and so does the rear edge position, based on its center at position 202. As a result, the mark length varies between the minimum mark length Lmin and the maximum mark length Lmax. When reading the information retained by the edge positions, a shift value in the edge positions is detected by observing a readout signal waveform at timings corresponding to both edges of the mark. Specifically, the levels of a readout signal waveform 409 are measured at timings of the edges 201 and 202 of non-modulated data, so that the positions of the front and rear edges can be detected by converting them into multi-levels.
In the method of recording information by varying the positions of the front and rear edges of a mark by integer multiples of specific interval Δ, if the edge-position variation interval Δ is narrowed in an attempt to increase areal density, stricter control precision at recording would be required and there would be less detection margin for the detection of readout signal multi-levels. Furthermore, if the number n of variations is increased while the edge-position variation interval Δ is fixed, the readout signal levels at both edges of non-nodulation data could be saturated or become zero and, as a result, the edge positions could not be detected beyond the dynamic range, making it difficult to increase areal density.